Because of easy processing, high safety, excellent mechanical and chemical properties, the epoxy resin has been widely used in the field of coating, electrical insulation, construction building material, adhesives and laminated entities. Particularly, since epoxy resins have strong adhesion to reinforcement material such as glass-fiber fabric, no volatility and small shrinkage of the forming product while hardening, a laminated plate produced by such resins has the advantage of a broad range of usability, good mechanical strength, good electrical insulation property, excellent resistance to chemicals and the like. The reliability of such a laminated plate has been increased, and such an epoxy resin laminated plate has been massively applied to electrical and electronic products.
In some applications, the flame retardant properties of a printed circuit board are absolutely necessary due to the safety of human life involved when the printed circuit board is used in traffic vehicles such as airplanes, automobiles and public transportation. In order to enhance the flame retardant properties of the laminated plate, substances that can isolate the flame and reduce burning should be used. For laminated plates of epoxy resin/glass-fiber series (or organic fiber series), halogen-containing compounds, especially bromine-containing epoxy resins and hardeners, are used in combination with flame retardants such as antimony trioxide and the like, so that the flame retardant properties of the laminated plates can reach the required standard (such as the UL 94V-0 grade). Generally, for reaching the UL 94V-0 standard, the epoxy resin containing bromine as high as 17 to 21% in combination with antimony trioxide or other flame retardant are used. For example, U.S. Pat. No. 5,262,491 disclosed a composition comprising polyphenylene oxide, epoxy resin, metal salt, and curing agents. The composition reached the standard of flame retardant properties and heat resistance, and had excellent dielectric properties at high temperature. However, this patent disclosed that the composition contains about 10% to 30% of bromine to reach the standard of flame retardant properties and heat resistance. However, the use of the flame retardant containing halogen, and antimony trioxide will seriously affect the health of humans.
On the other hand, since the demand for finer circuits and higher density of the printed circuit board is increasing day by day, it has been necessary to develop a laminated plate with better electrical, mechanical, and heat resistant processing properties. For FR4 laminated plate widely used at present, the glass transition temperature (Tg) after hardening is about 130° C. Thus, when the temperature is over 200° C. during cutting and drilling and even over 270° C. during the welding procedure in a printed circuit board process, the laminated plate breaks or cracks easily. The expansion dimension of FR4 laminated plate in plane is about 12 to 17 ppm/° C. For less than 100 μm of line width/line space of a developing printed circuit board, such a laminated plate is not suitably applied in HDI field. Therefore, various laminated plate materials that emphasize high heat stability and high glass transition temperature are constantly being developed.
Therefore, in the halogen-free resin composition of the present invention, a phosphorus-containing epoxy resin (particularly, a side chain type phosphorus-containing epoxy resin) and a hardener having the benzoxazine cyclic structure are used. The flame retardant properties of such a resin composition can reach the UL 94V-0 standard without adding the halogen-containing compound or resin, and such a resin composition has higher heat resistance relative to other resin composition comprising a conventional hardener. On the other hand, the halogen-free resin composition of the present invention comprises polyphenylene oxide resin so that the composition can have excellent dielectric properties in addition to flame retardant properties and heat resistance.
Generally, the compounds having the benzoxazine cyclic structure is prepared by the reaction of a phenolic compound, an amine compound and an aldehyde compound. But many patents disclose the method for preparing the compounds having the benzoxazine cyclic structure, which are prepared by the reaction of aniline and phenolic compound. For example, U.S. Pat. No. 6,005,064 disclosed the thermosetting resin having the benzoxazine cyclic structure prepared by the reaction of a phenolic resin, formaldehyde and aniline; and JP-A-Hei-11-50123 also disclosed the method for preparing dihydrobenzoxazine thermosetting resin from bisphenol, aniline and formalin and using methyl ethyl acetone as a solvent. However, aniline used in these preparing method is toxic, and is a forbidden chemical material by the law. The preparation method cannot meet the requirements with the mass production in industry.
Therefore, the present inventors have conducted extensive studies and have found that the system was endowed with relatively high stability as using the hydrocarbon solvent to conduct the reaction of phenolic compounds, aromatic diamines and aldehyde compounds. In addition to without using toxic aniline as a reactant, it can be prevented that the compounds having the benzoxazine cyclic structure conducts the ring-reopening polymerization at high temperature due to improper temperature control. Meanwhile, the present invention can prevent the gelation or agglomeration from being occurred caused by using high polar solvent or protic solvent to undergo the reaction. Therefore, the present invention is useful in mass production in industry.